1. Field of the Art
The present specification generally relates to the field of electronic paper devices. More particularly, the present specification relates to systems and methods for updating electrophoretic displays using bounding box based display driving control.
2. Description of the Related Art
Several technologies have been introduced recently that provide some of the properties of paper in a display device that can be updated electronically. Some of the desirable properties of paper that these types of display devices try to achieve include: low power consumption, flexibility, wide viewing angle, light weight, high resolution, high contrast, and readability indoors and outdoors. Because these display devices attempt to mimic the characteristics of paper, they are referred to as electronic paper devices (EPDs). These display devices are also referred to as: paper-like displays, zero power displays, e-paper displays, bi-stable displays or electrophoretic displays.
A comparison of EPDs to Cathode Ray Tube (CRT) displays or Liquid Crystal Displays (LCDs) reveals that in general, EPDs require less power and have higher spatial resolution; however, many EPDs have lower update rates, less accurate color control and lower color resolution than LCDs or CRTs.
In a conventional LCD, the luminance, or color, of a pixel depends on the voltage applied to the pixel, with a given voltage corresponding to a specific luminance. In contrast, the luminance or color of a pixel in an EPD typically changes based on the duration that voltage is applied to the pixel as well as the voltage value. For example, in some electrophoretic displays, applying a negative voltage to a pixel makes the pixel lighter (i.e., makes the pixel have a higher luminance) and applying a positive voltage makes the pixel darker. The higher the voltage and the longer or more frequently that voltage is applied, the larger the change in luminance. Hence, electrophoretic displays are typically controlled by applying a sequence of voltages to a pixel instead of applying a single voltage to a pixel, like a typical LCD. Often, a sequence of voltages applied to an electrophoretic display pixel is referred to as a “waveform.”
Additionally, control signals driving a pixel of an electrophoretic display also depend on the optical state to which the pixel is being driven and on the optical state from which the pixel is being driven. Other factors, such as temperature of the electrophoretic display, optical state of the pixel prior to the current optical state and the time since the pixel was last driven, are also taken into consideration when identifying a waveform to drive a pixel of an electrophoretic display.
Accordingly, conventional controllers for driving an electrophoretic display are often configured like an indexed color-mapped display. For example, a frame buffer of an electrophoretic display includes indices to a waveform used to update an image rather than the waveform itself. When the optical state of a pixel is to be changed, the index of the appropriate waveform is chosen based on one or more factors, such as current pixel state and/or destination pixel state and the pixel's location in the frame buffer is set to the index corresponding to the chosen waveform.
It generally takes longer to apply a waveform to modify a pixel of an electrophoretic display than it does to modify a pixel of a conventional CRT or LCD display. This can lead to noticeable latency between requests to display a new image on an electrophoretic display and when the electrophoretic display displays the new image. For example, an electrophoretic display using a conventional display controller may take 0.5 seconds to update a 1200×825 display. The latency can be reduced by simplifying the waveform calculation, for example by ignoring secondary factors such as dwell time and pixel history (prior displayed colors for the pixel) prior to the current optical state. However, such simplifications to waveform calculation result in remnants of prior displayed images remaining visible, a problem commonly referred to as “ghosting,” when the electrophoretic display is modified.
While current update times are generally sufficient for the page turning needed by electronic books, they are problematic for interactive applications such as pen tracking, or soft key typing. A user may tolerate waiting for a second or two for transitioning between two pages when the user spends a few minutes reading each page. However, if a user wants to write or type on a page with instant visual feedback, the increased latency of electrophoretic display update becomes unacceptable.